1. Field of the Invention
This invention relates to processes for selectively separating a mixture of components through sorption and desorption and, more particularly to a novel simulated moving bed apparatus and method whereby the simulated moving bed is created within an uninterrupted sorbent bed by imbedding strategically within such sorbent bed multiple distributors for injection of feedstock and eluent along with multiple collectors for removal of extract and raffinate and to obtain narrower fraction cuts by reducing the time for injecting feedstock and eluent and collecting separated fractions respectively without stopping the percolation of circulating fluid through the sorbent bed which results in significantly reduced bed compaction and flow restriction.
2. The Prior Art
The commercial application of column chromatography for the separation of dissolved constituents using suitable sorbents and batch operation has evolved over the years to a level as represented by Yoritomi et al (U.S. Pat. Nos. 4,379,751 and 4,267,054). To achieve a reasonable level of separation the sorbent beds in these systems must be relatively tall. Yoritomi specifies at least 10 meters. At such high bed depths the flow rates through the bed must be relatively low to avoid progressive bed compaction and eventual total blockage to flow. These necessary low flow rates restrict operating capacities which could potentially be available from state of the art, high kinetic separating mediums. The stop and go operation of a batch process as represented by U.S. Pat. No. 4,379,751 also leaves a large part of the separating medium idle in certain parts of the column while feedstock and eluent are added to the column or extract and raffinate fractions are withdrawn. Additionally, the total removal of certain concentration bands from the column liquid as practiced by U.S. Pat. No. 4,379,751 imposes rather sudden changes of concentration gradients which impairs general operating efficiency in terms of osmotic shock on the resin and the need to re-establish this concentration band in subsequent cycles which retards the speed of operation. These impediments have all but eliminated the batch process from consideration for commercial application in column chromatography.
The invention of the so-called simulated moving bed process by Broughton et al (U.S. Pat. No. 2,985,589) improves on the batch operation in its most sophisticated form by providing for the continuous circulation of fluids through multiple beds of sorbents. The sorbent beds are arranged as an endless loop with periodic advances to the next sorbent bed within the loop for inlet flows of feedstock and eluent and outlet flows of effluent fractions, respectively. This operation is also referred to as a pseudo moving bed process. One form of commercialization of this process includes discrete multiple sorbent beds vertically stacked on top of each other in the form of a tower as initially proposed by the foregoing reference as well as those of Ishikawa et al (U.S. Pat. No. 4,182,633); Odawara et al (U.S. Pat. No. 4,157,267); Ando et al (U.S. Pat. No. 4,405,455). Another approach is the use of multiple individual columns horizontally arranged as a train with the train operated as an independent closed loop and is taught by Schoenrock et al (U.S. Pat. No. 4,412,866) and Ando et a (U.S. Pat. No. 4,599,115). In the commercial separation of dissolved constituents by chromatography such a the fractionation of fructose from dextrose or the separation of sucrose from highly impure sugar solution such as molasses by ionic exclusion using the so-called simulated moving bed technique, it becomes necessary to establish and maintain a very specific concentration profile. This concentration profile is distributed, as a rule, over four or more sorbent beds as taught by the foregoing references to aid in optimizing the introduction and withdrawal of streams at strategic positions of the closed loop.
One or more of these sorbent beds within the endless loop is projected to represent a specific zone which in their most fundamental form are referred to as sorption, displacement, elution and rinse zones, respectively. Continuous circulation of the loop fluid around this endless loop train causes each of the zones to be periodically shifted to the sorbent bed next in line downstream. The objective is t maintain a steady state concentration profile which moves as a wave continuously around the looped train while introducing feedstock and eluent to the train at strategic locations and removing separated fractions from the circulation fluid thereby establishing a continuum.
General performance efficiency and steady state operation of the process depend primarily upon the following factors:
1. Accurate control of the correct circulating flow to maintain a steady state profile through the entire loop. PA1 2. Correct selection of influent and effluent cuts. PA1 3. Uniform cross sectional distribution and drainage of fluids entering and leaving the beds, respectively. PA1 4. Uniform cross sectional, downward movement of the circulation fluid through the sorbent beds with avoidance of channeling or net lateral flow. PA1 5. Distinction between hydraulic balance and internally generated pressure through the circulation pumps and separate control for each pressure function.
Conventionally the determination and control of the circulation flow rate remains generally undefined and left to speculation or experimentation. The patent of Schoenrock et al refers to a total liquid displacement volume as being given to provide the basis for establishing the circulation flow rate without defining the meaning of that terminology or how one arrives at that value. Other patents are mute on this point and leave the impression that this value is derived through trial and error. Although the teachings of U.S. Pat. No. 4,412,866 are very specific for correcting a given basic circulation flow rate with measured inflow and outflow rates, experience has shown that these corrections are not accurate and ineffective if the basic circulation flow rate is not accurately know.. The foregoing problems reduce the operating efficiency and the need for periodic manual corrections of the circulation flow rate. Because of its dynamic nature the pseudo-moving bed operation of the known prior art generates a continuously changing concentration profile of the dissolved components in the fluid percolating through the sorbent beds in terms of absolute concentration as well as the relative concentration of the dissolved solutes to each other.
These systems also teach and practice a continuous inflow of feedstock and eluent and respective outflows of separated fractions throughout the complete cycle This constraint requires compromises for selecting the positions to introduce feedstock and eluent as well as for withdrawing effluent fractions. A constant feedstock and eluent composition is thus introduced into and spread over a continuously changing concentration profile in the circulation fluid while continuously changing effluent concentrations are withdrawn. Such a processing strategy compromises the background concentration profile. Continuous or frequent monitoring of these concentration profiles is therefore essential to bracket the target concentrations for inlet and outlet positions in pseudo-moving bed separator loops to approach optimum operating performance for the system. To overcome this impediment it is common practice to increase the number of sorbent beds within a sorbent bed train and thereby gain access to smaller changes in the concentration gradients and sharper separation. Hence, the generally held conviction that an increasing number of discrete sorbent beds in a sorbent bed train improves the separating efficiency for pseudo moving bed systems. However, multiple bed trains such as the tower trains require a relatively short bed depth of less than 1 meter for each of its vessels to manage an extremely high pressure drop associated with high flow velocities required by multiple bed trains and which are particularly observed in certain parts of such trains. This high pressure drop is as a rule isolated to those beds where high solids concentrations accumulate and where the sorbent medium expands due to the desorbent action. One option to reduce this restriction for large commercial plants is to increase the diameter of the column and reduce the bed depth for each sorbent bed in the sorbent bed train. It is, however, also generally recognized by experts that it becomes increasingly more difficult to maintain the required uniform cross-sectional distribution, uniform cross-sectional collection and uniform cross-sectional downward movement for fluids in pseudo-moving bed sorbent trains as the ratio of sorbent bed diameter to the sorbent particle size increases. Hence, separation performances deteriorate as a rule with increasing column diameter. Because of the recognized restrictions in sorbent bed diameter the need for multiple trains in large commercial installations is associated with greatly increased costs. All these aforementioned impediments are associated with reduced operating efficiency, greatly increased costs, increased control complexity and increased pressure drop restrictions. The various systems proposed and currently in use represent trial and error compromises deviating more or less from the ideal state for achieving the objectives referred to above.
I have now discovered the means to approach the ideal state of efficiency and performance at greatly reduced costs, reduced complexity of valving and reduced process control needs. Problems associated with the control of pressures are also virtually eliminated.
In view of the foregoing, it would be an advancement in the art to provide a novel, pseudo-moving bed apparatus in a single vessel having the capability to more efficiently utilize the sorption characteristics of the sorbent material in the sorbent bed. It would also be an advancement in the art to provide an apparatus and method for obtaining narrower fraction cuts through the more efficient utilization of the sorption characteristics of the sorption bed through a carefully controlled pseudo moving bed in the sorption bed. It would also be an advancement in the art to provide a sorbent bed with all of the zones for sorption and desorption of the desired constituent from the fluid stream contained in a single, continuous sorbent bed. Such a novel apparatus and method is disclosed and claimed herein.